The US has expressed "deep concern" over Hong Kong's decision to prevent Harry Wu, a leading China activist, from entering the territory.

Mr Wu, a US citizen and outspoken critic of China's human rights record, was refused a visa to address a seminar in the lead up to the fifth anniversary of Hong Kong's return to Chinese sovereignty on 1 July.

It is important that Hong Kong's autonomy be maintained

US consulate general spokesman

A spokesman for the US consulate general in Hong Kong told BBC News Online: "We are deeply concerned that Hong Kong has declined to permit entry of an American citizen who wished to enter Hong Kong.

"We have conveyed this concern to senior levels of the Hong Kong government".

The US complaint came as another US citizen and China specialist, academic Perry Link, was detained for questioning at Hong Kong airport before being allowed to enter the territory.

Mr Link co-authenticated the so called "Tiananmen Papers" of allegedly official documents relating to the 1989 Tiananmen Square killings of pro-democracy protestors.

Jiang due

His detention by immigration department officials for questioning - highly unusual in Hong Kong - has sparked fears that the territory's Chief Executive Tung Chee-hwa is moving towards a more active policy of limiting criticism of China in Hong Kong.

Mr Tung, who was not democratically elected and is unpopular among voters, begins a second five-year term in office on 1 July.

China's President Jiang Zemin is due to attend a ceremony marking the anniversary of the transfer of power.

The ban on Mr Wu, combined with the sending home at the weekend of another US citizen, believed to be a member of the Falun Gong movement, may point to Hong Kong's anxiety to prevent anti-China protests during President Jiang's visit.